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Community and Q&A

Exterior insulation while re-siding with an interior poly VB

MrScottSmith | Posted in General Questions on

I’ve been reading a lot of articles and questions/answers from this site and many others in preparation for a re-siding project. I am located in South-East New Brunswick, Canada, our climate zones run different than the US, but I believe it would be similar to Bangor ME which is zone 6A.

To the best of my knowledge the wall construction (1977 build) is, from the outside in, vinyl siding, felt paper wrap, board sheathing, 2×4 walls, fiberglass batt insulation, 6mil poly VB, 1/2″ drywall.

Because of the low insulation value of the 2×4 wall construction, I would like to add exterior insulation while re-siding the house. In my research, there seems to be some concerns about creating a moisture problem when insulating the exterior of the house if there is an interior poly VB present. I am not planning on removing the drywall or poly VB. I read up on Roxul comfortboard IS, but based on feedback on this board there seems to be some issues with it in respect to compression when installing strapping causing a wavy siding install.

So what is the best approach for an exterior only renovation in this climate zone with a poly VB installed on the inside? Don’t add exterior insulation, or add XPS, EPS or MW rigid insulation? Or some other product I am unaware of?

Thanks for any feedback.

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  1. GBA Editor
    Martin Holladay | | #1

    My standard answer:

    Many energy experts have worried whether it's a good idea to install exterior foam on a house with interior polyethylene. Although it would be better if the poly wasn't there, the fact is that tens of thousands of Canadian homes with interior poly have been retrofitted with exterior rigid foam, and there haven't been any reports of widespread problems. According to building scientist John Straube, all indications show that these retrofits are "not so risky as most people think. These homes will probably be fine."

    That said, the installation of exterior foam is not advised on any home that has suffered wet-wall problems like leaking windows, condensation in stud cavities, or mold. If you plan to install exterior foam during a siding replacement job, keep an eye out for any signs of moisture problems when stripping the old siding from the walls. Investigate any water stains on housewrap or sheathing to determine whether the existing flashing was adequate.

    If there is any sheathing rot, determine the cause -- the most common cause is a flashing problem, but condensation of interior moisture is not impossible -- and correct the problem if possible. If you are unsure of the source of the moisture, hire a home performance contractor to help you solve the mystery.

    If your sheathing is dry and sound, I don't think you need to worry about adding exterior foam. Adding a rainscreen gap will certainly go a long way toward avoiding future moisture problems. Of course, it's important to be meticulous with your details when you are installing your new WRB and window flashing. It's also important to keep your interior relative humidity within reasonable levels during the winter. Never use a humidifier.

    To summarize, here are four caveats:

    1. Be sure that your foam is thick enough to keep the wall sheathing above the dew point in winter. Read more on this topic here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    2. When the siding is being removed, inspect the existing sheathing carefully for any signs of water intrusion, and correct any flashing or housewrap problems.

    3. Install rainscreen strapping so that there is a ventilated gap between the new exterior foam and the siding.

    4. Keep your interior humidity under control during the winter; if the interior humidity gets too high, operate your ventilation fan more frequently.

  2. MrScottSmith | | #2

    Almost immediately after I posted I came across your "standard" answer in other question. So with the results being that there aren't typically any issues, would xps be the best route to go?

  3. GBA Editor
    Martin Holladay | | #3

    I prefer polyiso to XPS. Read why -- and read a lot of other tips -- here:
    How to Install Rigid Foam Sheathing

  4. white_pine_vista | | #4

    We used the 2" roxul comfortboard IS exterior insulation. We just nailed it on by hand with cap nails over the studs (through plywood sheathing) 16" on center. That helped prop out the 1x4 strapping on an even plane. There was still some fiddling but it was not bad at all. Other people have used screws for 4" of roxul in a similar way (instead of squash blocks). Some people use an air nailer to put the strapping on for even compression and then go back and screw it in. We did use a frame of foam around our outie windows as it was easier to nail window flanges into and tape peel and stick to. We also used foam where higher compressive strength was suggested such as where awning roof is attached to wall. See

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